1. Field of the Invention
The present invention relates to, for example, an image forming apparatus such as a copy device, a printer or a facsimile, and more particularly to such an image forming apparatus that transfers an image on an image bearing member onto an intermediate transfer member and then transfers the image on the intermediate transfer member onto a transfer material.
2. Description of the Related Art
Heretofore, there is known such an image forming apparatus that transfers a toner image formed on an image bearing member using an electrophotographic technique onto a recording material and then fixes that unfixed toner image in order to obtain a permanent image on the recording material. Such apparatus is more widely used as a color-image forming apparatus as society has become more information oriented in recent years.
FIG. 5 shows an outline configuration of one example of a conventional electrophotographic full-color image forming apparatus. To accelerate a speed of outputting color images, this image forming apparatus has in itself a plurality of photosensitive members (i.e., image bearing members), each of which is used to form toner images sequentially, which are once multi-transferred on an intermediate transfer member and then transferred onto a recording material collectively.
As shown in FIG. 5, the present image forming apparatus has four image forming sections (image forming stations) of 10Y, 10M, 10C, and 10K for four colors of yellow, magenta, cyan, and black respectively and also an intermediate transfer belt 80 as transfer means and a fixing device 40 as fixing means.
The image forming sections 10Y, 10M, 10C, and 10K are each provided as a unit, together with photosensitive drums as image bearing members 70Y, 70M, 70C, and 70K respectively, around which are respectively arranged primary charging rollers 12Y, 12M, 12C, and 12K; laser exposure devices 13Y, 13M, 13C, and 13K; developing devices 14Y, 14M, 14C, and 14K; primary transfer rollers 54Y, 54M, 54C, and 54K; and cleaners 16Y, 16M, 16C, and 16K. The intermediate transfer belt 80 is disposed in contact with each of the photosensitive drums 70Y through 70K and stretched over three rollers of a drive roller 51, a tension roller 52, and a secondary transfer opposed roller 53, thus being driven in rotation in the direction indicated by an arrow b in the figure.
The photosensitive drums 70 (70Y-70K) are each uniformly charged on their surface by the primary charging rollers 12 (12Y-12K), to subsequently expose a color-separated image to light using the laser exposure devices 13 (13Y-13K), in order to form on the surface of the photosensitive drums 70 an electrostatic latent image which corresponds to an original. This latent image is developed by the developing devices 14 (14Y-14K) using minus toner, to form a toner image on the surface of the photosensitive drums 70.
The above-mentioned image forming operations are performed on each of the image forming sections 10Y through 10K at their respective predetermined timing points, thereby forming various colors of toner images on the photosensitive drums 70. These various colors of toner images are sequentially transferred onto the intermediate transfer belt 80 at each of the primary transfer sections opposed to the primary transfer rollers 54 (54Y-54K) (primary transfer), to once form on the intermediate transfer belt 80 a full-color image in which those four colors (yellow, magenta, cyan, and black) of toner images are superposed on top of each other.
Then, these four colors of toner images are collectively transferred using a secondary transfer roller 55 onto a recording material P fed at predetermined timing by a feed roller 20 (secondary transfer). The recording material P as finished by this transfer process is conveyed to the fixing device 40, where it is heated and pressured to fix the toner images.
As mentioned above, the full-color image forming apparatus with an intermediate transfer member collectively transfers four colors of toner images on the intermediate transfer member onto a recording material, thus being excellent in that it produces less misregister in color (color registration). Also, in contrast to a system that absorbs a recording material on a recording material bearing member such as for example a transfer belt or transfer drum and then conveys the material, to directly transfer onto the material each color of toner images formed on a photosensitive drum and superpose these toner images on the recording material, this system of using an intermediate transfer member need not absorb or convey the recording material but only needs to collectively transfer onto the recording material full-color toner images formed by rotating the intermediate transfer member such as for example an intermediate transfer belt, thus forming images regardless of the kind of recording material, such as an envelope, cardboard, etc., with no variations in color registration due to the thickness of the recording material employed.
For this reason, therefore, particularly such an image forming apparatus using an intermediate transfer member is widely used for the electrophotographic-type full-color image forming apparatuses.
The above-mentioned primary transfer system, however, usually needs complicated transfer bias control. To achieve good transferability in all of the image forming sections 10Y through 10K, larger constant-voltage biases must be set at more downstream side image forming sections to give a sufficient transfer current to all the image forming sections, thus making it necessary to apply transfer biases from a total of four high-tension power supplies each independently for each of the image forming sections.
This is because the intermediate transfer belt is gradually charged up as it sequentially passes the image forming sections so that various colors of toner images may be superposed and transferred thereon, thus causing effective impedance in the width direction of the intermediate transfer belt passing the transfer nip sections to be increased as the belt passes more downstream side image forming sections.